Course Topics

Introduction & Classification

Alcohols, phenols, and ethers are organic compounds derived from water structure:

Compound Type Definition General Formula
Alcohol When H of alkane is replaced by –OH group R–OH
Phenol When H of benzene ring is replaced by –OH group Ar–OH
Ether Linkage between two carbon atoms through oxygen atom R–O–R’

Classification of Alcohols:

Primary Alcohols

  • –OH group attached to primary carbon
  • Example: CH₃CH₂OH (Ethanol)
  • Oxidation gives aldehydes → carboxylic acids

Secondary Alcohols

  • –OH group attached to secondary carbon
  • Example: (CH₃)₂CHOH (Isopropanol)
  • Oxidation gives ketones

Tertiary Alcohols

  • –OH group attached to tertiary carbon
  • Example: (CH₃)₃COH (tert-Butanol)
  • Resist oxidation, undergo elimination
Memory Tip

1° alcohol → aldehyde → acid; 2° alcohol → ketone; 3° alcohol → no oxidation (elimination only)!

Reactivity of Alcohols

Key Concepts:

  • Oxygen in –OH group is sp³ hybridized
  • C–O and O–H bonds are highly polarized
  • Negative charge resides on oxygen atom
CRITICAL CONCEPT: Reactivity order depends on bond cleavage:
  1. C–O bond breaks: Tertiary > Secondary > Primary > CH₃OH
  2. O–H bond breaks: CH₃OH > Primary > Secondary > Tertiary

Nucleophile breaks C–O bond; Electrophile breaks O–H bond.

Oxidation Reactions:

Alcohol Type Reagent Product
Primary Alcohol K₂Cr₂O₇/H₂SO₄ (mild) Aldehyde
Primary Alcohol K₂Cr₂O₇/H₂SO₄ (strong) Carboxylic Acid
Secondary Alcohol K₂Cr₂O₇/H₂SO₄ Ketone
Tertiary Alcohol K₂Cr₂O₇/H₂SO₄ Alkene (elimination)
Memory Tip

Tertiary alcohols say: “No oxidation for me! I’ll eliminate and make an alkene instead!”

Important Reactions of Alcohols

1. Reaction with HX (Lucas Test): Distinguishes 1°, 2°, 3° alcohols

  • Tertiary: Oily layer immediately (room temp)
  • Secondary: Oily layer in 5-10 minutes
  • Primary: Oily layer only on heating

2. Iodoform Test:

  • Ethanol gives yellow crystals with I₂/NaOH
  • Methanol does NOT give iodoform test
  • Test for CH₃CH(OH)– group
CH₃CH₂OH + 4I₂ + 6NaOH → CHI₃ (yellow) + HCOONa + 5NaI + 5H₂O

3. Esterification:

  • Nucleophilic substitution at carbonyl carbon
  • –OH of acid replaced by –OR’ of alcohol
  • Acid-catalyzed reversible reaction

4. Dehydration:

  • With conc. H₂SO₄ at different temperatures:
  • At 170°C: Alkene formation
  • At 140°C: Ether formation
  • Methanol cannot dehydrate (no β-hydrogen)
Memory Tip

Lucas test: 3° (fast), 2° (medium), 1° (slow) – like race results!

Phenols – Properties & Reactions

Physical Properties:

  • Colorless crystalline deliquescent solid
  • Characteristic phenolic odor
  • MP: 41°C, BP: 182°C
  • Sparingly soluble in water (pink solution)
  • Poisonous – used as disinfectant
CRITICAL CONCEPT – Phenol Structure:

Simplest phenol is carbolic acid (benzenol) C₆H₅OH, first isolated from coal tar by Runge in 1834.

–OH group directly attached to benzene ring makes it different from alcohols.

Reactions due to –OH group:

Reaction Product Remarks
With NaOH/Na Sodium phenoxide Shows acidic nature
Esterification Phenyl esters Requires basic media
Reduction with Zn Benzene –OH group removed
Memory Tip

Phenol = Benzene ring + OH = Aromatic alcohol with special properties!

Reactions of Phenol – Benzene Ring

Phenol undergoes electrophilic substitution reactions easily due to –OH activating effect:

Reaction Conditions Product
Nitration Dil. HNO₃, room temp o- and p-nitrophenol
Conc. HNO₃, heat Picric acid (2,4,6-trinitrophenol)
Halogenation Br₂ water 2,4,6-tribromophenol (white ppt)
Sulphonation H₂SO₄, 15-20°C o- and p-phenol sulphonic acid
With Formaldehyde Condensation polymerization Bakelite (thermosetting plastic)
Memory Tip

Phenol + Br₂ water = Instant white ppt (test for phenol)!

Acidity Comparison

Relative Acidity Order:

Alcohols
(pKa≈16-18)
Water
(pKa=14)
Phenol
(pKa≈10)
Carboxylic Acids
(pKa≈5)

Why is phenol acidic?

  • Phenoxide ion is resonance stabilized
  • Negative charge delocalized over benzene ring
  • Makes proton loss easier than alcohols
CRITICAL CONCEPT: Phenol is acidic enough to react with NaOH but NOT with NaHCO₃ (weaker than carbonic acid).

Test: Phenol + NaOH → Soluble phenoxide; Phenol + NaHCO₃ → No CO₂ evolution

Compound Formula Approx. Kₐ pKₐ
Phenol Ar–OH 10⁻¹⁰ 10
Alcohols R–OH 10⁻¹⁶ – 10⁻¹⁸ 16-18
Carboxylic acids R–COOH 10⁻⁵ 5
Water H–OH 10⁻¹⁴ 14
Memory Tip

Acidity: RCOOH > Phenol > H₂O > ROH (Remember: Carboxylic acid strongest, alcohol weakest!)

Alcohol vs Phenol – Key Differences

Alcohols (R–OH)

  • –OH attached to alkyl group
  • Derivatives of alkanes
  • Colorless liquids (lower members)
  • Sweet smell, burning taste
  • Readily soluble in water
  • Less acidic (pKa 16-18)
  • Alkoxide ions – no resonance
  • Reactions: C–O & O–H bond breaks

Phenols (Ar–OH)

  • –OH attached to aryl group
  • Derivatives of benzene
  • Colorless crystalline solids
  • Characteristic phenolic odor
  • Sparingly soluble in water
  • Acidic (pKa ~10)
  • Phenoxide ions – resonance stabilized
  • Reactions: –OH group & benzene ring
CRITICAL CONCEPT: The key difference is resonance stabilization in phenoxide ions, making phenols acidic while alcohols are nearly neutral.
Memory Tip

Alcohol = Alkyl + OH = Neutral; Phenol = Phenyl + OH = Acidic (thanks to resonance)!

Applications & Importance

Practical Applications:

  • Ethanol: Beverages, fuel, solvent, antiseptic
  • Methanol: Antifreeze, formaldehyde production, fuel
  • Phenol: Disinfectants, bakelite production, aspirin synthesis
  • Isopropyl alcohol: Rubbing alcohol, disinfectant
  • Ethylene glycol: Antifreeze in automobiles
  • Glycerol: Cosmetics, explosives, sweetener
  • Ethers: Anesthetics (diethyl ether), solvents
CRITICAL CONCEPT: The properties of alcohols and phenols (solubility, boiling points, reactivity) make them invaluable in pharmaceuticals, fuels, plastics, and everyday products.
Memory Tip

From hand sanitizer (alcohol) to disinfectants (phenol) to plastics (bakelite) – these compounds are everywhere!